It's a Bird,
It's a...Dinosaur?

In 1989 a fossil hunter scouring the limestone formations of the
south-central Pyrenees in Spain found the mineralized remains of an unusual
ancient bird. Now in the hands of paleontologists, those fossil bones are heating
up an already vigorous evolutionary debate: Are birds the direct evolutionary descendants of
dinosaurs? Is "the lost world" not lost at all, but hidden all around us
under a cloak of feathers?

Seeking answers to those questions, Jose L. Sanz of the Autonomous University
of Madrid analyzed the limestone relic to understand its relation to modern
birds. In a paper published in the June 6, 1997, issue of Science,
Sanz and his colleagues describe the primitive bird as an intriguing blend of the
old and the new.

Like the famed Archaeopteryx,
generally recognized as the first true bird, the new fossil has teeth in its jaw.
Like modern birds, it has well-articulated wings that probably could have
supported sophisticated flapping motions; although the specimen is 135 million
years old, its flying abilities were probably significantly superior to those of
Archaeopteryx, which lived just 10 million years earlier. And yet some
aspects of the skull--in particular, the bone formations behind the eyes--closely
resemble those of theropods
(small, bipedal dinosaurs), not birds.

Sanz's analysis turned up another surprise: the fossil bird appears to be a
hatchling, the most ancient one yet found. The surface pattern of its bones looks
much like those of today's hatchlings. Based on this and other evidence, the
researchers surmise that 135 million years ago, the early growth patterns of
birds had already been set. The hatchling's many birdlike qualities, in sharp
contrast to its primitive, dinosaurlike head, provide "additional data
solidifying the notion that modern birds are short-tailed, feathered descendants
of theropod dinosaurs," Sanz and his colleagues write.

This analysis comes on the heels of the announcement of another "missing
link" between dinosaurs and birds. Fernando E. Novas of the Argentine Museum
of Natural Science in Buenos Aires and Pablo F. Puerta of the Paleontological
Museum in Trelew, Argentina, recently announced the discovery of a new species
whose morphology suggests an intermediate step on the evolutionary path toward
birds. Novas and Puerta describe the small creature, which stood perhaps one
meter tall, in the May 22 issue of Nature.
They give it the provocative name Unenlagia, meaning "half bird" in the
language of the Mapuche Indians.

In most ways, Unenlagia was clearly a dinosaur--there is no evidence
that it had feathers, for instance. But it bears a closer resemblance to a bird
than any other dinosaur yet studied, the scientists claim. They point in
particular to the bone structure of the creature's arms, which could be folded in
much the way that birds tuck in their feathered wings. Unenlagia could
also elevate its forearms in a manner similar to the upstroke that birds use for
flight. >Lawrence M. Witmer
of Ohio University in Athens, Ohio, describes the mix of features "a true mosaic,
begging the question of where to draw the line between what is, or is not, a
'bird.' "

Although Unenlagia could not fly, its fossil remains contain some
fascinating clues about how flight might have evolved. Birds did not start out as
gliders like pterosaurs,
Novas and Puerta believe. Rather they began as flappers whose beating wings
co-opted motions that originally evolved to perform some other functions.
Speculating on what those functions might have been the authors propose that
Unenlagia "used its arms not only in predation but probably also in
maintenance of balance and to control its body attitude while running and
leaping." Interestingly, Sanz's hatchling fossil seems to tell the same story.
Its advanced, flapping wings already show close kinship with modern fliers. Other
aspects of bird anatomy, in comparison, took many millions of years to fall into
place.

DISSENTER. Alan
Feduccia of the University of North Carolina at Chapel Hill believes the fossil
evidence really shows dinosaurs and birds to be descendants of a common
ancestor.

All this talk of missing links infuriates some other scientists, who consider
the link between dinosaurs and birds overblown and misguided. Chief among these
dissenters is Alan
Feduccia of the University of North Carolina at Chapel Hill, whose recent
book, The Origin and Evolution of Birds [Yale University Press, 1996],
contains extensive refutation of the notion. Linking birds and dinosaurs plays
well given the big brutes' immense popularity, but Feduccia believes the fossil
evidence really shows dinosaurs and birds to be descendants of a common
ancestor--not unlike the relation between dinosaurs and mammals such as
ourselves.

Feduccia points out that Unenlagia is definitely not an intermediate
species between dinosaur and bird. It lived only 90 million years ago, 55 million
years after Archaeopteryx. Novas counters that Unenlagia is
probably the descendent of earlier birdlike dinosaurs and so represents a "sister
taxon" to Avialae,
the group that includes the modern birds. But as Feduccia noted last year in a
Science paper written with several co-authors, nearly every dinosaur
described as birdlike dates from long after the appearance of the first birds,
leading to what the researchers dub a "temporal paradox."

So is there a little T. rex in that Thanksgiving turkey? Or are
paleontologists being pulled by the same cultural fads that saddled our world
with Barney? The truth is out there, perhaps locked away in fossils yet to be
found.